Just a shot in the dark, but there's only one place you can have your cash where it will be available for trade. For example it's easy to think that any money market is cash for the purpose of investable assets, but other than their default sweep money market, everything else is considered a mutual fund, so for example if you put all of your cash in a different money market that's not their default, they would consider that an investment position (vs available investable cash) so you have to cash out of that before you can buy anything else. Again a shot in the dark and I may be completely off base but I've had this happen before.
I don't think that's true based on my limited, recent experience with a Fidelity brokerage account I funded several weeks ago. I funded the account with a wire into the account from a bank account. The cash was parked in the default core position, SPAXX, for this taxable brokerage account, which by the way is the same core position for my Roth IRA with Fidelity, but different from the core position for my Fidelity HSA, which is FDRXX.
For my taxable brokerage account which had all my cash in the core holding of SPAXX, I moved all of the cash into FMPXX, which showed up as available cash in that account. FMPXX, which required a minimum buy of $1 million was yielding around 35 basis more than SPAXX. A month later I purchased T-bills from this account, and FMPXX funds were immediately available as cash for this purchase; several weeks later I purchased VTI and likewise funds from FMPXX were immediately available for this purchase.
Unlike my experience at Wells Fargo Advisors (Well Trade Account), where a high yield money market fund, like WMPXX, is treated like a mutual fund and not available cash and you have to sell funds in WMPXX and reduce it to cash before trading into another position replacing those funds, Fidelity appears to treat its money market funds as cash to be used immediately for trades or other transfers.
Moreover, funds in my core position in my HSA, FDRXX, are now being partially liquidated on a monthly basis to pay for my LTCi premiums through ACH transfers. And I have used those funds to reimburse myself for prior, eligible HSA medical expenses!
I could be wrong about this because my experience is very limited and new.
In general, this explains the core position at Fidelity.
https://www.fidelity.com/learning-center/investment-products/mutual-funds/core-position-video